Currently, third generation (3G) mobile communication systems are being standardized to implement efficient and high throughput of downlink (DL) packet data transfer mechanisms. In the context of universal mobile telecommunications system (UMTS) wideband code-division multiple access (W-CDMA) based 3G systems, these packet transfer techniques are commonly referred to as high-speed downlink packet access (HSDPA). HSDPA is possible for both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes, and is implemented for chip rates of 1.28 Mcps and 3.84 Mcps.
The following characteristic features are the source of recognized efficiency and achievable data throughput of HSDPA: Adaptive Modulation and Coding (AMC) techniques; fast Hybrid Automatic Repeat Request (Hybrid ARQ); fast feedback mechanism for uplink (UL) reporting of instantaneous DL channel quality; and radio resource efficient packet scheduling mechanism and fast, short-term DL channel assignments.
Yet another distinguishing feature of HSDPA is that the data rate and amount of DL transmission (Tx) power that an HSDPA base station allocates to a wireless transmit and receive unit (WTRU) is a function of the WTRU's instantaneous channel conditions. For example, a user close to the base station can reliably receive a high HSDPA data rate with low transmission power. A user far away from the base station, or a user confronted with unfavorable channel conditions, would only support a reduced data rate for the same or higher amount of allocated DL transmission power.
The instantaneous HSDPA data rate that a particular user can reliably support is generally dependent on: 1) path loss, which is based on the distance to the serving base station; 2) shadowing; 3) instantaneous fast fading conditions; 4) interference at the user's receiver caused by other users present in the system; and 5) the user's channel conditions, such as the speed and propagation environment. In other words, the HSDPA data rate is a function of the user's experienced DL signal-to-interference ratio (SIR) which is based upon all of these factors and is representative of the DL data rate that the user can support. The user's DL SIR will generally vary in time as a function of these factors.
Knowledge of the user's experienced DL SIR or any similar representative metric with this functionality, for example BLER, BER, or received signal power combined with received DL interference, is essential for the HSDPA base station to ensure highly efficient HSDPA operation. CDMA systems employing HSDPA have therefore adopted a fast UL Layer 1 (L1) signaling mechanism which allows a WTRU to periodically report the DL SIR to the base station with a fast, UL channel quality indicator (CQI). The current FDD specification allows configuration of the periodic CQI feedback in the UL to be sent every 0 (when CQI reporting is turned off), 2, 4, 8, 10, 20, 40, 80, or 160 ms. However, in TDD there is no periodic CQI feedback, so the CQI is instead sent with an ACK/NACK on the High-Speed Shared Control Channel (HS-SICH) whenever a DL data block on the HSDPA Data Channel (HS-DSCH) is received by a WTRU. In W-CDMA FDD and TDD modes, this mechanism is commonly referred to as CQI reporting.
The method for measuring a CQI in a particular WTRU implementation is not standardized, but is open to vendor implementation. But the method of how to derive the reported CQI value is standardized. In the FDD standard, there is a table (as shown in 3GPP TS 25.321, Medium Access Control (MAC) Protocol Specification, 5.4.0 (2003-03)) listing some 30 CQI values roughly corresponding to increasingly higher data rates, and therefore proportional to higher and higher DL SIRs. The reported CQI in FDD is derived as follows (per 3GPP TS 25.214, Physical layer procedures (FDD), v5.4.0 (2003-03), section 6A.2): “the UE shall report the highest tabulated CQI value for which a single HS-DSCH sub-frame formatted with the transport block size, number of HS-PDSCH codes and modulation corresponding to the reported or lower CQI value could be received in a 3-slot reference period ending 1 slot before the start of the first slot in which the reported CQI value is transmitted and for which the transport block error probability would not exceed 0.1.” In TDD, the reporting is different; the transport block size is reported if it was transmitted during the last received transmission interval (the number of timeslots where the last HS-DSCH was received) and that transmission would have yielded a block error rate of 0.1.
As an example, in the current W-CDMA FDD release 5, the CQI is an information bit sequence five bits long which is encoded by means of a (20, 5) Reed-Muller code. The resulting 20 bit long coded sequence is sent in the UL on a High-Speed Dedicated Physical Control Channel (HS-DPCCH). Every user has a separate HS-DPCCH with an adjustable CQI reporting cycle (feedback rate). A user can report the CQI on the HS-DPCCH even if the user does not receive data on the HS-DSCH.
As another example, in the current W-CDMA TDD release (3.84 Mcps or high chip rate (HCR) TDD), the CQI is an information bit sequence ten bits long which is encoded by means of a (32, 10) Reed-Muller code. The resulting 32 bit long coded sequence is sent in the UL as part of the HS-SICH. With current TDD, a CQI transmission can only take place if the user has previously received data on the HS-DSCH in the frame.
Because the reliability of a WTRU's CQI report has an impact on HSDPA operation, it is important that an HSDPA base station has a means of determining whether a CQI was received in error. By discarding any erroneously received CQI, the HSDPA base station can avoid the situation in which it would choose a DL data rate and corresponding transmission power for a user that is not adapted to the user's experienced DL channel conditions. Erroneous CQIs reduce the HSDPA data throughput to the user and create a high level of interference to the other users in the system, which reduces the efficiency of HSDPA service in the W-CDMA system.
Furthermore, too many CQIs received in error from a particular user are an indication that the user's UL transmission power settings are not accurate and the base station or another access network node, such as the Radio Network Controller (RNC), will take appropriate actions. As an example, the RNC can signal a higher target UL SIR to the user in order to increase its UL transmission power and to lower the error rate on HS-DPCCH (in FDD) or HS-SICH (in TDD). This type of RNC functionality is commonly referred to as outer loop power control.
Error detection of received UL transmissions in W-CDMA FDD and TDD modes is typically accomplished by employing a Cyclic Redundancy Check (CRC), i.e., a bit sequence computed from and accompanying the data which, when decoded in error in the base station, is a reliable indicator of decoding errors. For a CRC to be effective in error detection, the length of the CRC must be sufficiently large. However, in order to avoid having an inefficient process, the ratio of the CRC length to the actual data length must be small. In a typical application, the data may be on the order of hundreds of bits while the CRC field may be on the order of 8-24 bits.
Unfortunately, the HS-DPCCH (FDD) and the HS-SICH (TDD) are fast L1 UL signaling channels which do not include any UL data or a sufficient number of L1 signaling bits to make efficient use of a CRC. To provide sufficient error detection capabilities, the CRC would have to be nominally at least the same size as the data field that it is verifying. With these considerations, current HSDPA standards do not use a CRC on the HS-DPCCH (FDD) and the HS-SICH (TDD).
Therefore, based on existing techniques the network (base station or RNC) has no means of reliably determining whether or not a CQI was received in error or not. The network can only configure the WTRU to use a high enough UL transmission power by means of an UL target SIR and by “experience” from simulations, such that the event of errors is sufficiently unlikely and not detrimental to HSDPA system operation. It is therefore advantageous to provide a method for reliably detecting and reporting the correctness of received CQI values.